Residual toxicity of Black Walnut
A very large (4 foot diameter) Black Walnut was just removed in a back yard and the stump was ground to a depth of 3 feet. How long will the decomposing roots cause toxicity in the soil before it is safe to install new plantings in the immediate area?
Montgomery County Maryland
As soon as the tree is cut, the decomposition process begins. Juglone (the toxic chemical produced by black walnut trees) in the soil will decompose in a couple of months. Warm, moist soil will promote greater micro-organism activity. Dry weather and cold slows or stops it.
Woody parts of the tree will, of course, break down more slowly (for example, mulch from the tree should be given 6 months--at least.) Because there will be some larger roots in the soil, you might want to give the site more than two months. On the other hand, when planting you can just remove all the roots you encounter as you go along, and that will speed up how soon you can plant.
Here is the juglone page from our website: http://extension.umd.edu/hgic/walnut-wilt-juglone-trees-and-shrubs